Ab Initio Calculations on Intermolecular Forces. III. Effect of Electron Correlation on the Hydrogen Bond in the HF Dimer

The influence of electron correlation on the linear hydrogen bond in F-H · · · F-H is studied in the frameworks of the -, CEPA-, and PNO-CI approaches. Due to compensating effects of the intrasystem and the intersystem correlation energy we find a relatively small net effect of the total correlation energy on the interaction energy, the F-F distance, and the intermolecular vibration. The following R_ff and ΔE values (in au) result in the SCF-, -, CEPA-, and PNO-CI approach respectively: 5.48, —0.0055; 5.47, —0.0054; 5.46, —0.0056; 5.43, —0.0056. The intermolecular stretching vibration of the H-F bond involved in the hydrogen bond is hardly affected in comparison with the isolated HF molecule. The experimentally observed shifts of 10-15% in the absorption frequencies of infrared spectra are thus attributed to higher polymers, in agreement with previous theoretical and experimental works.